Diseases involving the retinal vasculature are one of the leading causes of blindness worldwide. Many of these diseases are both progressive and treatable. Thus, their early detection is highly desirable. Diagnoses are often made on the basis of the many obvious structural changes which may occur in the retina as a consequence of problems with retinal blood flow. These include neovascularization (the growth of new blood vessels in an attempt to compensate for a reduction in flow through pre-existing vessels), cotton-wool patches (regions in which nerve fiber axoplasmic transport has failed), and eventually the degeneration of retinal nerve fibers. Once observed, these and other phenomena may be used to diagnose retinal vascular disease, and treatment may begin to inhibit further degeneration. However, it is desirable to detect such problems early, if possible, before irreversible damage has occurred. Thus, attention has focused on developing methods of diagnosing retinal vasculature problems by measuring the rate of retinal blood flow, a reduction of which occurs prior to later, more serious, problems.
U.S. Pat. No. 6,588,901 to Grinvald et al., which is incorporated herein by reference, describes a system for directly imaging and analyzing the movement of individual erythrocytes in blood vessels. The system includes imaging means for acquiring, within a predetermined time interval from each other, at least one pair of images of at least one same erythrocyte for producing at least two frames, each image representing an analog or digital image of the location of the erythrocyte in each of the frames at a predetermined time. The system also includes image acquisition means for collecting and storing analog or digital images in machine-readable form, and a computer for controlling the operation of the imaging means and the image acquisition means, for processing the at least two frames, and for analyzing the movement of the erythrocyte in the blood vessels. A method for directly imaging and analyzing the movement of individual erythrocytes in blood vessels is also described.
An article by Michelson G et al., entitled, “Flickering light increases retinal blood flow,” Retina, 22(3):336-343, June 2002, which is incorporated herein by reference, describes the examination of retinal blood flow in normal eyes before and during retinal stimulation by flickering light. Laser Doppler flowmetry measurements are described as having been synchronized with the electrocardiogram, in order to decrease the influence of the heartbeat on pulsation of retinal blood flow. As described, only phases similar in terms of the systolic or diastolic phase within the heart cycle were compared.
An article by Grinvald A. et al., entitled, “In-vivo optical imaging of cortical architecture and dynamics,” published in Modern Techniques in Neuroscience Research, U. Windhorst and H. Johansson (eds.), Springer Verlag, which is incorporated herein by reference, describes imaging the brain, and various techniques for reducing the effect of heartbeat-induced and respiration-induced motion of the brain.